The present invention is generally directed to a device for generating a solid particulate aerosol, and particularly to such a device capable of accurately dispensing a predetermined quantity of a solid particulate material within a unit quantity of a gaseous material.
It is desirable in many industrial applications to generate a solid particulate aerosol. Examples where such aerosols are desirable include the powder painting industry where metallic objects are electrically charged and then exposed to a solid particulate aerosol consisting of oppositely charged pigment particles. The pigment particles are electrically attracted to the charged metallic objects, thereby coating such objects with a uniform pigment layer.
Conventional solid particulate aerosol generators, such as those presently used in the powder painting industry, comprise a carrier gas which is caused to flow through a venturi, the throat of which venturi is in fluid communication with a particulate reservoir. As the carrier gas passes through the venturi, particulate matter from the reservoir is drawn by vacuum into the throat of the venturi where it is mixed with the carrier gas. However, conventional particulate aerosol generators of the type described above do not work well where the particulate matter is relatively small (mean average diameter less than about 0.2 microns) because small particles tend to form irregular clumps which, when dispersed by conventional generators, form aerosols having widely fluctuating particle concentrations. Conventional solid particulate aerosol generators also do not work well where the particle concentration of the aerosol needs to be maintained with high precision. Conventional aerosol generator are simply not capable of high precision.
An additional drawback to convention solid particulate aerosol generators is that such generators are not particularly flexible in that they tend to produce a very limited range of particle concentrations. Increasing the volume of carrier gas through the venturi tends to increase the vacuum formed in the venturi throat which, in turn, tends to increase the quantity of solids drawn into the carrier gas stream.
A specific area where conventional solids particulate aerosol generators are especially deficient is in the testing of high efficiency particulate air filters (HEPA filters), such as the filters used in hospitals, clean rooms and electronic assembly areas. The testing of HEPA filters using a solid particulate aerosol is described in U.S. Pat. No. 4,213,768, which patent is incorporated by reference herein as if fully set forth in its entirety. As described in U.S. Pat. No. 4,213,768, HEPA filters must be frequently tested to assure their continued efficiency. Testing with a solid particulate aerosol has marked advantages over testing with liquid aerosols, but the testing of these ultra-efficient filters with a solid particulate aerosol requires exposing the filters to an aerosol having a very specific concentration of ultra-fine particles (generally such particles must be less than about 0.2 microns). Conventional solids particulate aerosol generators are incapable of producing such a specifically concentrated aerosol. The ultra-fine nature of the test particles cannot be accurately metered into the carrier gas. The resulting aerosols have concentrations which fluctuate over a wide range, making any calculation of the filter's efficiency impossible.
There is therefore a need for a solid particulate aerosol generator which can accurately disperse particles having a mean average diameter smaller than about 0.2 microns.
There is a further need for a solid particulate aerosol generator which can produce aerosols with a highly precise concentration of solids material.
There is a still further need for a solid particulate aerosol generator which can adjustably produce aerosols having a wide variety of solids concentrations.
There is a still further need for a solid particulate aerosol generator which can produce a highly specific concentration of ultra-fine particles so as to be useful in the testing of HEPA filters.